1. Field of the Invention
The present invention relates to an antenna means for a portable radio communication device, comprising a radiating first element tuned to a first frequency, the first element having a central longitudinal first axis, first and second ends being a first feed point and a first open end, respectively, and a meander configuration. Specifically, the invention concerns an antenna means for a hand-portable mobile telephone, which requires a compact and efficient antenna. The inventive antenna means is particularly advantageous when two or more radiating elements are to be combined or when an impedance matching means is required for matching radiating element(s) of the antenna means to transmitter/receiver circuitry of the telephone.
A general problem that occurs when the size of an antenna radiator is reduced is a reduction in its relative bandwidth. Helically configured radiators are commonly used when antennas are required to fit in confined volumes with limited height. However, the loops of a helical antenna generate a magnetic field that binds energy, which results in a further reduction of the bandwidth. Also, helical radiators have the problem of strong inter-coupling when two or more radiators are arranged close to each other.
2. Prior Art
GB-A 2 280 789 discloses an antenna means having multiple turns formed by a conductive radiating element formed on a dielectric substrate. The substrate may be tubular having conductive strips on one side, the strips being joined together along meeting edges of the tubular substrate. In another embodiment, the substrate is flat and has conductor strips deposited on both sides, the strips being joined together by feed-throughs along opposite edges of the substrate. That prior art antenna device has the inherent drawbacks of helical antennas, and is difficult and complicated to manufacture because of the need to provide feed-throughs in the substrate or joining conductors at edges.
Although relatively efficient and compact, there is a great variety of prior art antenna devices that involve the above-mentioned problems due to the use of helical radiators. Such antenna devices are disclosed in, for example, European Patent Applications published under Nos. 0 635 898 A1, 0 593 185 A1, and 0 467 822 A2, PCT Applications published under Nos. WO 94/10720 and WO 95/08199, and U.S. Pat. No. 4,868,576.
In the past, meander antennas have been used when an antenna device is required to have a total length which is short in relation to the wavelength at which an associated transmitter/receiver is operated. DE-A1 31 29 045 discloses a direction finder antenna having, for example, a meander structure. A radiating element thereof has a meandering configuration and is mounted on a dielectric carrier.
DE-A1 31 29 045 is considered to disclose the prior art antenna closest to the invention. The problem to be solved thereby is reducing the height of a direction finder antenna, in particular to render it concealable and mobile However, it only discloses a meander antenna which has a flat configuration. Moreover, the teachings thereof include improving the bandwidth of the antenna by using a conductor having relatively high resistance, leading to a less efficient antenna. Further, there are no provisions for obtaining a mechanically durable antenna, an antenna suited to fit in a limited volume or an antenna to be combined with other types of antennas.
Another plane meandering antenna element is disclosed in Abstracts of Japan 60 E 1572 (publication No. 6-90108), and includes a meandering dipole and a matching means connected to a coaxial transmission line. A meandering feed arrangement for a helical antenna is disclosed in U.S. Pat. No. 5,298,910. In none of the latter two devices, a transmission line is connected to an end of the meandering conductor.
The pending Swedish Patent Application No. 9601706-6 includes means integrated with the antenna for matching the antenna to circuitry of a hand-portable mobile telephone. A similar matching means is suitable also in the present invention. The above-mentioned Swedish Patent Application is therefore incorporated herein by this reference.
An object of the invention is to provide an efficient antenna means for a portable radio communication device, comprising a radiating first element tuned to a first frequency, the first element having a central longitudinal first axis, first and second ends being a first feed point and a first open end, respectively, and a meander configuration, which solves the problem of providing an antenna means that is mechanically durable and has a geometry suited for location in a small volume. Further objects are to provide substitutes for helical radiators which also give improved antenna performance, to overcome the above-mentioned problem of binding electromagnetic energy in the radiator or radiators of the antenna means, to avoid feed-throughs in a carrier carrying the radiating element(s), to provide an efficient and cost-effective impedance matching means integrated with the antenna means, to provide a configuration which is both efficient and mechanically durable, to enable the use of more precise production techniques that, e.g., wound helices, and to provide an antenna wherein different radiating elements may be combined without being adversely inter-coupled, especially wherein the combination includes an extendable whip antenna.
These and other objects are attained by an antenna means in which the first element alternately extends in positive and negative angular directions in relation to the first axis. This radiator geometry has been found to be particularly advantageous with regard to stability, bandwidth and radiating properties. The radiating first element of this antenna means is a meandering conductor which is arched or bent so that it will occupy a space similar to that occupied by a helical radiating element. This configuration enables the antenna means of the invention to be used in most application in radio communication devices, especially for mobile telephones, where helical antennas have been used in the past. In comparison with a helical antenna, the advantages of using the antenna device of the invention are, for example, a greater bandwidth, improved production tolerances leading to less rejections, a lower degree of coupling to any adjacent radiators greatly improving multi-band operability, and a possibility to integrate an impedance matching network on the same carrier with at least partly the same production technique. The radiating element alternately extending in positive and negative angular directions in relation to its central axis, should be understood as including the radiating element describing a meander curve changing circumferential direction at least once in its extension along a longitudinal axis of an imaginary cylindrical shell, preferably having a circular or elliptic base.
When the antenna means includes one or more additional radiating element(s), operability within a wider frequency band or two or more separated frequency bands is achieved. It is possible to produce all radiating elements simultaneously in the same sequence of process steps.
When restriction of the electromagnetic energy bound in the radiating structure is specifically important, it does not include any complete turns at all and, preferably, it may only include configurations describing small fractions of a full turn around a central axis.
The first and second feed points may be interconnected and coupled in common to circuitry of the radio communication device. This could also be applied when using more than two radiating elements. Alternatively, the different radiating elements may be connected separately to the radio circuitry.
The antenna device preferably includes a dielectric carrier carrying the radiating structure to project it outwards from a chassis of a radio communication device on which the device is to be mounted. This enables an efficient radiation pattern. The carrier is preferably a dielectric flexible film or laminate having the radiating structure applied thereon or therein in the form of a conductive film structure, possibly obtained through an etching process. A printing technique is suitable for manufacturing in large quantities.
It may be advantageous to combine the antenna means according to the invention with an extendable and retractable whip antenna, as will be appreciated from the following description of preferred embodiments. The carrier and conductors of the antenna means will then possibly include one or more switches for connecting or disconnecting different radiating elements in different operating modes.
Especially when the carrier is a flexible film with a printed circuit pattern it is advantageous to integrate on the carrier an impedance matching means for matching impedances of any radiating element on the film or in combination with that structure to circuitry of the radio communication device, usually interfacing at 50 ohms.